Bash – Documentation for Line Continuations After && and ||

bashshell-script

I have seen this construct in scripts a lot and used it myself, but it bothers me that I can't seem to find it in the documentation.

Example:

[ -f file1 ] &&
[ -f file2 ] &&
echo "Both files exist." ||
echo "One or the other file doesn't exist."

This could also be done with backslashes before the newlines, as mentioned in man bash:

If a \<newline> pair appears,  and  the  backslash  is  not
itself  quoted,  the \<newline> is treated as a line continuation (that
is, it is removed from the input stream and effectively ignored).

Example:

[ -f file1 ] && \
[ -f file2 ] && \
echo "Both files exist." || \
echo "One or the other file doesn't exist."

…but this doesn't seem to be necessary. The first version above works even without the backslashes.

Where can I find this in man bash? (Also, is this bash specific or POSIX compliant?)

Best Answer

A newline is ignored in a few contexts where there is manifestly an unterminated command. These contexts include after a control operator (&&, ||, |, &, ;, ;;, but not !).

I don't see this documented in the bash manual.

In POSIX, it's specified via the grammar rules. Wherever the rules have linebreak, you can have zero or more line breaks.

Related Solutions

Bash – Logical AND in a bash script

If you want either then you want OR, not AND.

if [[ ! -f /etc/benchmarking/code ]] || [[ ! -f /etc/benchmarking/code.class ]]; then
 echo "match"
fi

This will match if either or both files are missing.
Your code will only print match if both do not exist.

But, you said:

I am 110% sure that these 2 files do not exist. I don't get any errors, it just doesn't enter the if.

So your statement contradicts itself. At least one of those files must exist, if you are running that code.

If you want to see how your if statement is evaluating, run it with -x.

#!/bin/bash -x
if [[ ! -f /etc/benchmarking/code ]] && [[ ! -f /etc/benchmarking/code.class ]]; then
 echo "match"
fi

Then you'll see the execution.

$ ./test.sh
+ [[ ! -f /etc/benchmarking/code ]]
+ [[ ! -f /etc/benchmarking/code.class ]]
+ echo match
match
$

Bash – How to Add Arguments to ‘bash -c’

You're interpreting the man page wrong. Firstly, the part about -- signalling the end of options is irrelevant to what you're trying to do. The -c overrides the rest of the command line from that point on, so that it's no longer going through bash's option handling at all, meaning that the -- would be passed through to the command, not handled by bash as an end of options marker.

The second mistake is that extra arguments are assigned as positional parameters to the shell process that's launched, not passed as arguments to the command. So, what you're trying to do could be done as one of:

/bin/bash -c 'echo "$0" "$1"' foo bar
/bin/bash -c 'echo "$@"' bash foo bar

In the first case, passing echo the parameters $0 and $1 explicitly, and in the second case, using "$@" to expand as normal as "all positional parameters except $0". Note that in that case we have to pass something to be used as $0 as well; I've chosen "bash" since that's what $0 would normally be, but anything else would work.

As for the reason it's done this way, instead of just passing any arguments you give directly to the command you list: note that the documentation says "commands are read from string", plural. In other words, this scheme allows you to do:

/bin/bash -c 'mkdir "$1"; cd "$1"; touch "$2"' bash dir file

But, note that a better way to meet your original goal might be to use env rather than bash:

/usr/bin/env -- "ls" "-l"

If you don't need any of the features that a shell is providing, there's no reason to use it - using env in this case will be faster, simpler, and less typing. And you don't have to think as hard to make sure it will safely handle filenames containing shell metacharacters or whitespace.

Best Answer

Related Solutions

Bash – Logical AND in a bash script

Bash – How to Add Arguments to ‘bash -c’

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